Intra- and interspecific agonistic behaviour in sympatric harriers during the breeding season

Abstract

We analysed the temporal and sexual patterns of intra- and interspecific aggression in sympatric harriers during the breeding season, to determine the main resource defended (food, nest sites, mates) and how factors such as body size or breeding system (territorial versus colonial) influence aggressive behaviour. We predicted that if aggression is (at least partly) related to competition for food, the hen harrier, Circus cyaneus, because of its large size and territorial system, should be more aggressive (both intra- and interspecifically) than the smaller, colonial Montagu's harrier, Circus pygargus. The intraspecific aggression rate of both harriers peaked early in the season, was mainly intrasexual and increased with the number of neighbours. These patterns support the mate competition hypothesis to explain intraspecific aggression in both species. Montagu's harriers were more aggressive towards conspecifics than hen harriers. Their aggression rates towards heterospecifics were high at the start of the season then decreased, supporting the hypothesis that interspecific aggression serves primarily for nest site defence. Hen harriers showed lower interspecific aggression rates in the prelaying period and a gradual increase throughout the breeding cycle, particularly by females, who hunt around the nest for food for the nestlings. These patterns correspond to food competition, as food resources around the nest are probably more important and interspecific intrusion more costly for territorial hen harriers than for colonial Montagu's harriers. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.